A Study Of Acoustic Mode Coupling And Mechanism Of Interference Striations In R-omega Plane

To invert ocean environment by means of sound field is an important acoustical problem. While the premise is to study the rule of sound propagation , offering valuable information for inverting ocean environment.This thesis researches the mechanism of acoustic mode coupling in inhomogeneous water volume and primarily analyzes the interference of the acoustic intensity as a function of source range and frequency.It is known that mode coupling occurs when acoustic wave propagates through inhomogeneous water volume. Usually, mode coupling between normal modes is solved by two-way coupling mode, one-way coupling mode and adiabatic mode theories. In this thesis, we adopt Evans's coupled mode theory and apply the perturbation theory with the change of phase shift to derive an integral expression for mode coupling scattering-matrix. This provides a new approach for the mode coupling problem. This expression can depict physical image of normal mode's coupling frankly, combined with the corresponding physical interpretation. The intensity of sound field changes only as function of source frequency and range when the location of sound source and receiver is fixed. Therefore, contour plots of acoustic intensity are mapped in range and frequency, and show remarkably "coherent pattern" or striations due to the interference between normal modes. They are so-called R-omega Plane.In a range-independent ocean waveguide, a scalar parameter called waveguide invariant β , is useful for interpretation of interferencepatterns in R-ωplot. By use of the perturbation method, we derived an expression for the waveguide invariant. The expression may be useful for ocean environment whichis horizontally slowly varying. Adiabatic approximation is used in the derivation.In addition to typical striations, there are specific interference patterns such as "branched-line","splited-line" and "isolated-line" in the range and frequency planes. To study these, some numerical calculations are performed. Different interference terms are gradually superimposed, variations of phase and (horizontal) interference wave length of interference terms with the change of frequency are investigated in details. Based on these observations, physical explanations are presented for these specific interference patterns.